Skip to content

MEMORY_LAYOUT.md

Latest commit

 

History

History
132 lines (109 loc) · 4.8 KB

MEMORY_LAYOUT.md

File metadata and controls

132 lines (109 loc) · 4.8 KB

UnoDOS Memory Layout

Physical Memory Map (Real Mode, 640KB)

Address Range          Size    Usage
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
0x0000:0x0000          1 KB    Interrupt Vector Table (IVT)
0x0000:0x0400          256 B   BIOS Data Area (BDA)
0x0000:0x0500          ~30 KB  FREE / Transient Program Area
0x0000:0x7C00          512 B   Boot Sector (loaded by BIOS)
0x0000:0x7E00          ~1 KB   FREE (after boot sector)
0x0800:0x0000          2 KB    Stage2 Bootloader
0x1000:0x0000          28 KB   Kernel
  0x1000:0x1A00        ~364 B    API dispatch table (105 functions)
0x1400:0x0000          ~64 KB  Heap (malloc pool)
0x2000:0x0000          64 KB   Shell/Launcher segment (fixed)
0x3000:0x0000          64 KB   User app slot 0 (dynamic pool)
0x4000:0x0000          64 KB   User app slot 1
0x5000:0x0000          64 KB   User app slot 2
0x6000:0x0000          64 KB   User app slot 3
0x7000:0x0000          64 KB   User app slot 4
0x8000:0x0000          64 KB   User app slot 5
0x9000:0x0000          64 KB   Scratch buffer (window drag content)
0x9FFF:0xFFFF          -       End of conventional memory
0xA000:0x0000          64 KB   EGA/VGA graphics memory
0xB000:0x0000          32 KB   Monochrome text memory
0xB800:0x0000          16 KB   CGA text/graphics memory
0xC000:0x0000          256 KB  ROM BIOS extensions
0xF000:0x0000          64 KB   System BIOS ROM

Kernel Layout (0x1000:0x0000)

Offset    Content
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
0x0000    Signature ("UK") + entry point
0x0002    Kernel code (init, drivers, API stubs)
          - INT 0x80 handler
          - Keyboard driver (INT 09h)
          - PS/2 mouse driver (INT 0x74)
          - Graphics routines (pixel, rect, string, XOR)
          - Memory allocator (malloc/free)
          - Event system
          - FAT12 filesystem driver
          - Application loader with segment pool
          - Window manager (create, destroy, draw, focus, move, drag)
          - Mouse cursor (XOR sprite, hide/show, drag state)
          - Content preservation (save/restore scratch buffer)
          - Segment pool (alloc_segment, free_segment)
0x1100    API table (header + 105 function pointers)
~0x1084   Font data (8x8 + 4x6)
~0x1500   Data section (window_table, app_table, segment_pool, variables)
~0x6FFF   End of 28KB kernel (padded)

API Table Structure (at 0x1000:0x1A00)

Offset  Size  Content
0x00    2     Magic: 0x4B41 ('KA')
0x02    2     Version: 0x0100 (1.0)
0x04    2     Function count: 91
0x06    2     Reserved
0x08    182   Function pointers (105 × 2 bytes)

Segment Architecture

Why Kernel at 64KB Boundary?

Segment:Offset = 0x1000:0x0000
Linear address = (0x1000 × 16) + 0x0000 = 0x10000 = 64KB

Reasons:

  1. Clean Segmentation - Single segment (0x1000) covers entire kernel
  2. No segment manipulation - DS, ES, CS all 0x1000 during kernel execution
  3. Clear heap boundary - Heap at 0x1400:0000 is 16KB after kernel start
  4. Future compatibility - Aligned for potential protected mode transition

Multi-App Segment Architecture (Build 149+)

0x2000:0x0000  Shell (Launcher) - fixed, persists always
0x3000:0x0000  User app slot 0 (dynamic pool)
0x4000:0x0000  User app slot 1
0x5000:0x0000  User app slot 2
0x6000:0x0000  User app slot 3
0x7000:0x0000  User app slot 4
0x8000:0x0000  User app slot 5
  • Shell loads at 0x2000 (fixed), user apps allocated dynamically from pool
  • Up to 6 concurrent user apps, each in its own 64KB segment
  • alloc_segment(task_handle) finds free segment, marks owned
  • free_segment(segment) returns segment to pool on task exit
  • segment_pool (6 words) + segment_owner (6 bytes) track allocation state
  • Apps use [ORG 0x0000] and are loaded at offset 0

Scratch Buffer (0x9000)

Shared kernel buffer used for system services:

  • 0x0000-0x0FFF: System clipboard (4KB, used by clip_copy/clip_paste APIs 84-86)
  • 0x1000+: File dialog list buffer (used by file_dialog_open API 90)
  • Size: up to 64KB
  • Moved from 0x5000 to 0x9000 in Build 149 to free segment for app pool

Current Memory Usage

Region Size Usage Notes
0x0500-0x7C00 30KB Unused (TPA) Available for future use
0x1000-0x16FF 28KB Kernel In use
0x1400-0x1FFF 48KB Heap Available for malloc
0x2000-0x2FFF 64KB Shell segment Launcher (fixed)
0x3000-0x8FFF 384KB User app pool 6 × 64KB segments
0x9000-0x9FFF 64KB Scratch buffer Clipboard (4KB) + file dialog list

Overall: 7 app segments (1 shell + 6 user) using 448KB of the 640KB address space.

v3.23.0 Build 397